Brown spots on green leaves

I've got a bunch of nice, healthy plants in my grow room, but lately one has been getting brown spots on her leaves. It's just one plant, all my other ones look fine. They're five weeks into flowering, in Happy Frog soil. The only thing that stands out about this plant is that it's my biggest one that's planted in a 1.5 gallon pot. I know 1.5 gallons is on the small side, but my plant is less than 3 feet tall and at this point it's too late to transplant so I've been careful about watering every day with a good nutrient mix.

Other than these brown spots, the plant looks quite healthy, with nice sticky buds.

I've been using Jack's Classic fertilizer and I've been feeding using the directions on the container for every time you water. I started with the 20-20-20 mix, then added in flowering fertilizer (10-30-20) as flowers appeared. Now I'm using 100% flowering fertilizer.

Here's some pictures of one of the worst leaves. No, there are no spider mites. Any ideas?

looks like thats happening to my plants, unlucky for me mine are only 6 days old

Check this place
http://www.marijuana-seeds.net/Thanks-ThankYou.htm

I would use less nutes... flush thier system out a lil

Yeah, I've got another page that has all those same leaf pictures. I wish they'd get some better-quality pictures, it seems they've been using the same ones forever.

It's probably just nute burn though. I've been feeding my plants quite a lot. The only thing is, only the one plant is showing problems and it's my biggest one. I've been feeding all the plants about the same. You'd think the big guy would use more nutes and thus not be the first to show signs of nute burn. I will try flushing though.